Moved AudioError enum inside audio.c

The only thing I did not change was the _g for globals. Is there any
other way we can mark globals?

This is a first step toward a bigger project. Some modules could be
ported to header-only to be used as standalone.

Checking some files to be converted to header-only

Tint color could be applied to colDiffuse... but what's the best way?
Replace it? Multiply by? A point to think about...

Functions renamed to improve library consistency

Useful to save small data files (configuration and so)
For bigger files, external data storage should be used (SDCard)

It tints all fragments, ignores lighting. Useful for some features like
feedback (damage color, ...).

Scaling normal depth (y axis) makes disappear the specular of
fragments... So I think it can be removed, it is not a very
useful/important attribute.

Draw3DCircle is useful to draw point lights radius.

Support render texture size different than screen size

Now shader maps use a generic naming convention for any kind of texture
maps (not only diffuse, normal or specular). Useful for custom shaders.

- 3 light types added (point, directional, spot).
- DrawLights() function added using line shapes.
- Standard lighting example added.
- Removed useless struct variables from material and light.
- Fixed light attributes dynamic locations errors.
- Standard vertex and fragment shaders temporally added until rewrite it
as char pointers in rlgl.
TODO:
- Add normal and specular maps calculations in standard shader.
- Add control structs to handle which attributes needs to be calculated
(textures, specular...).
- Adapt standard shader to version 110.
- Rewrite standard shader as char pointers in rlgl.

In standard shader, material glossiness is a value from 0 to 1000 like
in MLT files. So, it doesn't need to be normalized.

- Ambient and lambert lighting added.
- Ambient and diffuse colors linked to standard shader.
- Single light linked to standard shader.
- LoadStandardMaterial() and depending functions added.

...on model drawing

Material glossiness is a float type value...

Improved materials

Only works when raylib is installed on windows system.

_g naming convention for globals, new error exit numbers.